1. Technical Field
This disclosure relates to linear roller bearing assemblies, roller-bearing sub-assemblies, and reciprocating machinery incorporating the same, more particularly in Stirling engines and cryocoolers.
2. Description of the Related Art
Free-piston Stirling engines and cryocoolers include linear bearings to guide and support components such as pistons which reciprocate within a pressurized gas environment. Differential gas pressures across the piston are sealed by a narrow, non-contact annular gap (clearance seal). It is desirable to maintain concentricity of this gap to maximize performance and efficiency by minimizing energy losses due to gas leakage through said gap. These bearings should prevent contact between the piston and cylinder as it reciprocates so as to avoid wear and energy loss. Rubbing of the piston would reduce the machine's life and performance due to wear, and contamination of both the working gas and the internal heat exchangers. The bearings must support the lateral and rotational loads. The bearings should not require lubrication that would contaminate the working gas and thus reduce performance and efficiency.
Flexural spiral springs and/or gas bearings are currently used to provide the long life linear bearings for these types of machines. Commonly used spiral flexure bearings (see e.g. FIG. 6 of U.S. Pat. No. 5,647,217) are limited in their stroke capability, frequency of operation, and radial load capacity. Gas bearings are also used in these types of machines. They can be expensive to manufacture, due to the extremely close dimensional tolerances required. Wear can occur during start-up and shut-down when contact is made between their mating surfaces. These bearings can lock up if a small foreign particle becomes lodged between these precision surfaces. Simple gas bearings do not provide rotational restraint that is usually required by linear alternators or motors.
Linear roller bearings have been used to guide linear movement of one surface relative to another. For example, U.S. Pat. No. 3,932,045 (the '045 patent) directed primarily to rolling contact joints useful as prosthetic joints, shows a linear bearing in FIGS. 14-16. The linear bearing includes two rollers and straps wrapped tightly around each roller. One problem with the linear bearing shown in FIGS. 14 and 15 of the '045 patent is that, over time, the straps can loosen around the rollers. The rollers can therefore slide laterally or slip rotationally within the straps to degrade the performance of the linear bearing. While the linear bearing shown in FIG. 16 of the '045 patent appears to include edge guides disposed on opposite ends of the rollers, the rollers can additionally rub against these guides if the straps loosen around the rollers. This rubbing can cause undesirable wear and friction. With the potential for undesirable loosening of the straps, lateral slide and rotational slip of the roller, and an undesirable amount of wear and friction during operation, the linear bearing of the '045 patent is not suitable for use in machinery such as Stirling engines and cryocoolers having components which reciprocate at high frequencies.
U.S. Pat. No. 7,240,580 (the '580 patent) is understood to disclose the use of linear bearings within a Z-axis positioner. Specifically, linear bearings guide movement of a stage along a single axis relative to a base. The straps in these linear bearings can be attached to a roller, or simply wrapped around the roller. However, the stage does not reciprocate at the frequencies encountered by components of machines such as Stirling engines and cryocoolers. Also, movement of the stage appears to be limited to approximately one quarter of the circumference of the roller due to the configuration of the Z-axis positioner. Such limited movement is not desirable within machines such as Stirling engines and cryocoolers where relatively long stroke length is desired to achieve power or work output.
Accordingly, a need remains for a better linear bearing suitable for Stirling engines, cryocoolers and like applications.